Thermo-electrochemical converter

ABSTRACT

A thermo-electro-chemical converter direct heat to electricity engine has a monolithic co-sintered ceramic structure or a monolithic fused polymer structure that contains a working fluid within a continuous closed flow loop. The co-sintered ceramic or fused polymer structure includes a conduit system containing a heat exchanger, a first high density electrochemical cell stack, and a second high density electrochemical cell stack.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/503,255 titled “Thermo-Electrochemical Converter”, which is a Section371 of International Application No. PCT/US2015/044435, filed Aug. 10,2015, which was published in the English language on Feb. 18, 2016,under International Publication No. WO 2016/025372 A1, and which claimspriority to U.S. Patent Application No. 62/035,560, filed Aug. 11, 2014,the disclosures of which are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

The conversion of heat energy or chemical energy to electrical energy,or visa-versa, may be accomplished in a variety of ways. For example,known electrochemical cells or batteries rely on chemical reactionswherein ions and electrons of a reactant being oxidized are transferredto the reactant being reduced via separate paths. Specifically, theelectrons are transferred electrically via wiring through an externalload where they perform work and the ions are conducted through anelectrolyte separator.

However, battery type electrochemical cells can produce only a limitedamount of energy because the confines of the battery casing limit theamount of available reactants that may be contained therein. Althoughsuch cells can be designed to be recharged by applying a reversepolarity current/voltage across the electrodes, such recharging requiresa separate electrical source. Also, during the recharging process, thecell is typically not usable.

Fuel cells have been developed in an effort to overcome problemsassociated with battery type electrochemical cells. In conventional fuelcells, the chemical reactants are continuously supplied to and removedfrom the electrochemical cell. In a manner similar to batteries, fuelcells operate by conducting an ionized species through a selectiveelectrolyte which generally blocks passage of electrons and non-ionizedspecies.

The most common type of fuel cell is a hydrogen-oxygen fuel cell whichpasses hydrogen through one of the electrodes and oxygen through theother electrode. The hydrogen ions are conducted through the electrolyteseparator to the oxygen side of the cell under the chemical reactionpotential of the hydrogen and oxygen. Porous electrodes on either sideof the electrolyte separator are used to couple the electrons involvedin the chemical reaction to an external load via an external circuit.The electrons and hydrogen ions reconstitute hydrogen and complete thereaction, while the oxygen on the oxygen side of the cell results in theproduction of water which is expelled from the system. A continuouselectrical current is maintained by a continuous supply of hydrogen andoxygen to the cell.

Mechanical heat engines have also been designed and used to produceelectrical power. Such mechanical heat engines operate on thermodynamiccycles wherein shaft work is performed using a piston or turbine tocompress a working fluid. The compression process is performed at a lowtemperature and, after compression, the working fluid is raised to ahigher temperature. At the high temperature, the working fluid isallowed to expand against a load, such as a piston or turbine, therebyproducing shaft work. A key to the operation of all engines employing aworking fluid is that less work is required to compress the workingfluid at low temperatures than that produced by expanding it at hightemperatures. This is the case for all thermodynamic engines employing aworking fluid.

For example, steam engines operate on the Rankine thermodynamic cycle,wherein water is pumped to a high pressure, and then heated to steam andexpanded through a piston or turbine to perform work. Internalcombustion engines operate on the Otto cycle, wherein low-temperatureambient air is compressed by a piston and then heated to very hightemperatures via fuel combustion inside the cylinder. As the cyclecontinues, the expansion of the heated air against the piston producesmore work than that consumed during the lower temperature compressionprocess.

The Stirling engine has been developed to operate on the Stirling cyclein an effort to provide an engine that has high efficiency and offersgreater versatility in the selection of the heat source. The idealStirling thermodynamic cycle is of equivalent efficiency to the idealCarnot cycle, which defines the theoretical maximum efficiency of anengine operating on heat input at high temperatures and heat rejectionat low temperatures. However, as with all mechanical engines, theStirling engine suffers from reliability problems and efficiency lossesassociated with its mechanical moving parts.

In an effort to avoid the problems inherent with mechanical heatengines, Alkali Metal Thermo-Electrochemical Conversion (AMTEC) cellshave been designed as a thermo-electrochemical heat engine. AMTEC heatengines utilize pressure to generate a voltage potential and electricalcurrent by forcing an ionizable working fluid, such as sodium, throughan electrochemical cell at high temperatures. The electrodes couple theelectrical current to an external load. Electrical work is performed asthe pressure differential across the electrolyte separator forces moltensodium atoms through the electrolyte. The sodium is ionized uponentering the electrolyte, thereby releasing electrons to the externalcircuit. On the other side of the electrolyte, the sodium ions recombinewith the electrons to reconstitute sodium upon leaving the electrolyte,in much the same way as the process that occurs in battery and fuel celltype electrochemical cells. The reconstituted sodium, which is at a lowpressure and a high temperature, leaves the electrochemical cell as anexpanded gas. The gas is then cooled and condensed back to a liquidstate. The resulting low-temperature liquid is then re-pressurized.Operation of an AMTEC engine approximates the Rankine thermodynamiccycle.

Numerous publications are available on AMTEC technology. See, forexample, Conceptual design of AMTEC demonstrative system for 100 t/dgarbage disposal power generating facility, Qiuya Ni et al. (ChineseAcademy of Sciences, Inst. of Electrical Engineering, Beijing, China).Another representative publication is Intersociety Energy ConversionEngineering Conference and Exhibit (IECEC), 35th, Las Vegas, Nev. (Jul.24-28, 2000), Collection of Technical Papers. Vol. 2 (A00-37701 10-44).Also see American Institute of Aeronautics and Astronautics, 190, p.1295-1299. REPORT NUMBER(S)— AIAA Paper 2000-3032.

AMTEC heat engines suffer from reliability issues due to the highlycorrosive nature of the alkali metal working fluid. AMTEC engines alsohave very limited utility. Specifically, AMTEC engines can only beoperated at very high temperatures because ionic conductive solidelectrolytes achieve practical conductivity levels only at hightemperatures. Indeed, even the low-temperature pressurization processmust occur at a relatively high temperature, because the alkali metalworking fluid must remain above its melt temperature at all times as itmoves through the cycle. Mechanical pumps and even magneto-hydrodynamicpumps have been used to pressurize the low-temperature working fluid.

In an effort to overcome the above-described drawbacks of conventionalmechanical and thermo-electrochemical heat engines, the JohnsonThermo-Electrochemical Converter (JTEC) system (disclosed in U.S. Pat.No. 7,160,639 filed Apr. 28, 2003) was developed. Referring to FIG. 2,there is shown a typical JTEC system (electrical connections not shown).JTEC is a heat engine that includes a first electrochemical cell 100operating at a relatively low temperature, a second electrochemical cell110 operating at a relatively high temperature, a conduit system 112including a heat exchanger 114 that couples the two cells together, anda supply of ionizable gas (such as hydrogen or oxygen) as a workingfluid contained within the conduit system. Each electrochemical cellincludes a Membrane Electrode Assembly (MEA).

More particularly, the JTEC heat engine includes a first MEA stack 118coupled to a high temperature heat source Q_(H) (i.e., a hightemperature MEA), a second MEA stack 116 coupled to a low temperatureheat sink Q_(L) (i.e., a low temperature MEA), and a recuperative heatexchanger 114 connecting the two MEA stacks 116, 118. Each MEA stack116, 118 includes a non-porous membrane 120 capable of conducting ionsof the working fluid and porous electrodes 122 positioned on oppositesides of the non-porous membrane 120 that are capable of conductingelectrons.

MEAS have been used in the fuel cell community to generate power viaelectrochemical reactions involving a fuel and an oxidizer, such ashydrogen and oxygen. However, the MEA stacks in conventional fuel cellapplications require bidirectional flow in at least one of theelectrodes. For example, oxygen flow into the cathode side ofhydrogen-oxygen fuel cells must be maintained as the same time that thehydrogen-oxygen reaction product, water, is exiting. As such, large flowcross-sections for fuel and the oxidizer/reaction product must be aninherent feature of the design of conventional MEA stacks for fuelcells.

No such bidirectional flow is required in the JTEC. Specifically, duringoperation of the JTEC, the working fluid passes through each MEA stack116, 118 by releasing an electron to the electrode 122 on the enteringside, such that the ion can be conducted through the membrane 120 to theopposite electrode 122. The working fluid is reconstituted within theopposite electrode 122 as it re-supplies electrons to working fluid ionsas they exit the membrane 120. The low temperature MEA stack 116operates at a lower voltage than the high temperature MEA stack 118. Thelow temperature MEA stack 116 compresses the working fluid at lowvoltage and the high temperature MEA stack 118 expands hydrogen at highvoltage. The difference in voltage between the two MEA stacks 116, 118is applied across the external load. The hydrogen circulatescontinuously inside the JTEC heat engine and is never consumed. Thecurrent flow through the two MEA stacks 116, 118 and the external loadis the same.

Specifically, in the JTEC heat engine, a hydrogen pressure differentialis applied across each MEA stack 116, 118 with a load attached, therebyproducing a voltage and current as hydrogen passes from high pressure tolow pressure. The electron current is directed to the external load aselectrons are stripped from the protons as they pass through themembrane 120, which is a proton conductive membrane (PCM). The JTECsystem utilizes the electrochemical potential of hydrogen pressureapplied across the PCM 120. More particularly, on the high pressure sideof MEA stack 116 and the low pressure side of MEA stack 118, hydrogengas is oxidized resulting in the creation of protons and electrons. Thepressure differential at the high temperature end forces the protonsthrough the membrane 120 causing the electrodes 122 to conduct electronsthrough an external load, while the imposition of an external voltageforces protons through the membrane at the low temperature end. On thehigh pressure side of MEA stack 116 and the low pressure side of MEAstack 118, the protons are reduced with the electrons to reform hydrogengas.

Unlike conventional fuel cells, in which the hydrogen exiting the MEAstack would encounter oxygen and react with it producing water, there isno oxygen or water in the JTEC system. This process can also operate inreverse. Specifically, if current is passed through the MEA stack 116, alow-pressure gas can be “pumped” to a higher pressure. The reverseprocess is rather similar to that of using a MEA stack to electrolyzewater, wherein water molecules are split and protons are conductedthrough the PCM, leaving oxygen behind on the water side. Hydrogen isoften supplied at a high pressure to a pure hydrogen reservoir via thisprocess.

In the JTEC, using hydrogen as the ionizable gas (i.e., the workingfluid), the electrical potential due to a hydrogen pressure differentialacross the PCM 120 is proportional to the natural logarithm of thepressure ratio, and can be calculated using the Nernst equation:

$\begin{matrix}{{V_{OC} = {\frac{RT}{2F}{\ln\left( \frac{P_{H}}{P_{L}} \right)}}},} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where V_(OC) is open circuit voltage, R is the universal gas constant, Tis the cell temperature, F is Faraday's constant, P_(H) is the pressureon the high pressure side, P_(L) is the pressure on the low pressureside, and P_(H)/P_(L) is the pressure ratio. E.g., Fuel Cell Handbook,J. H. Hirschenhofer et al., 4^(th) Edition, p. 2-5 (1999).

The voltage generated by the MEA stack 116 is thus given by the Nernstequation. The voltage is linear with respect to temperature and is alogarithmic function of the pressure ratio. FIG. 1 is a plot of theNernst equation for hydrogen and shows the voltage vs. temperaturerelationship for several pressure ratios. For example, referring to FIG.1, at a pressure ratio of 10,000, when the temperature is relativelyhigh, the voltage is similarly relatively high and when the temperatureis relatively low, the voltage is similarly relatively low.

The working fluid in the JTEC is compressed in the low temperatureelectrochemical cell 100 by supplying current at a voltage that issufficient to overcome the Nernst potential of the low temperature cell100, thereby driving hydrogen from the low pressure side of the membrane120 to the high pressure side. On the other hand, the working fluid isexpanded in the high temperature electrochemical cell 110 as current(power) is extracted under the Nernst potential of the high temperaturecell 110. Electrical current flow is generated as hydrogen expands fromthe high pressure side of the membrane 120 to the low pressure side. Asin any thermodynamic engine employing a working fluid and consistentwith the nature of compressible gas, in the JTEC, a greater amount ofwork (electricity) is extracted during high temperature expansion thanthe work (electricity) input required for the low temperaturecompression. The difference in heat energy input to the engine tomaintain constant temperature during high temperature expansion versusthe heat energy removed to maintain constant temperature during lowtemperature compression is provided as the difference in electricalenergy output by the high temperature expansion process versus thatconsumed by the low temperature compression process.

Consistent with the Nernst equation, the high temperature cell 110 willhave a higher voltage than the low temperature cell. Since the current(I) is the same through both cells 100, 110, the voltage differentialmeans that the power generated through the expansion of hydrogen in thehigh temperature cell 110 is higher than that of the low temperaturecell 100. The power output by the high temperature cell (V_(HT)*I) issufficient to drive the compression process in the low temperature cell100 (V_(LT)*I) as well as supply net power output to an external load((V_(HT)*I)−(V_(LT)*I)). This voltage differential provides the basisfor the JTEC engine.

Operation of the JTEC is generally similar to any other engine. Forexample, in a typical jet engine, the compressor stage pulls in air,compresses the air, and supplies the compressed air to the combustionchamber. The air is then heated in the combustion chamber and expandsthrough the power stage. The power stage couples shaft work back to thecompressor stage, in order to maintain a continuous supply of compressedair. The difference in work generated by the power stage and thatconsumed by the compressor stage is the net work output by the engine.However, the primary difference between such conventional engines andthe JTEC is that such conventional engines utilize a turbine (i.e., amechanical device) and operate on the Brayton thermodynamic cycle,whereas the JTEC is an all solid-state engine that operates on the moreefficient Ericsson cycle, which is equivalent to the Carnot cycle.

Referring to FIG. 3, there is shown the ideal temperature entropydiagram for the Ericsson engine cycle of the JTEC. Reference numerals“1” through “4” in FIGS. 2-3 represent different thermodynamic states.The thermodynamic states 1 through 4 are identical at the respectiveidentified points in FIGS. 2 and 3. As shown in FIG. 2, beginning at thelow-temperature, low-pressure state 1, electrical energy W_(in) issupplied to the low-temperature MEA stack in order to pump hydrogen fromthe low-temperature, low-pressure state 1 to the low-temperature,high-pressure state 2. The temperature of the hydrogen is maintainednearly constant by removing heat Q_(L) from the PCM 120 during thecompression process. The membrane 120 is relatively thin (i.e., lessthan 10 μm thick), and thus will not support a significant temperaturegradient, so the near isothermal assumption for the process is valid,provided adequate heat is transferred from the membrane 120 through itssubstrate.

From state 2, the hydrogen passes through the recuperative, counterflowheat exchanger 114 and is heated under approximately constant pressureto the high-temperature state 3. The heat needed to elevate thetemperature of the hydrogen from state 2 to 3 is transferred fromhydrogen flowing in the opposite direction through the heat exchanger114. At the high-temperature, high-pressure state 3, electrical power isgenerated as hydrogen expands across the MEA stack 118 from thehigh-pressure, high-temperature state 3 to the low-pressure,high-temperature state 4. Heat Q_(H) is supplied to the thin filmmembrane 120 to maintain a near constant temperature as the hydrogenexpands from high-pressure state 3 to low-pressure state 4. From state 4to state 1, the hydrogen flows through the recuperative heat exchanger114, wherein its temperature is lowered by heat transfer to hydrogenpassing from state 2 to 3. The hydrogen is pumped by the low-temperatureMEA stack 100 from state 1 back to high-pressure state 2 as the cyclecontinues.

However, some challenges have been encountered with developing a JTECthat is suitable for widespread use, particularly for systems that usehydrogen as the working fluid. For example, hydrogen leakage throughsmall defects in the conduit system may occur due to the small size ofthe hydrogen molecule. In particular, hydrogen leakage can occur at thejoints of the interconnects for the conduit couplings between thehigh-temperature cell and the low-temperature cell.

The engine design is also complicated by the need for a largemembrane/electrode surface area and by the need for a significant numberof cells to be electrically connected in series to achieve practicaloutput voltage levels. Specifically, unlike conventional fuel cells,where the open circuit voltage can be greater than 1V, the Nernstvoltage from the hydrogen pressure differential across a MEA stack is inthe range of only about 0.2 Volts. As such, many cells will have to beconnected in series to achieve useful output voltage levels.

Further, in order to achieve efficient energy conversion, the membranesmust have high diffusion barrier properties, because diffusion ofworking fluid (such as hydrogen gas) under the pressure differentialacross the membrane results in reduced electrical output and efficiency.The membranes utilized must also have good ion conductivity. However,known and available membrane materials that have good ion conductivity,such as Nafion manufactured by the DuPont Corp., generally have verypoor molecular diffusion barrier properties. Conversely, known andavailable membrane materials that have high molecular diffusion barrierproperties generally have relatively low ionic conductivity, and use ofsuch materials would result is high system impedance and highpolarization losses. As such, large membrane areas are needed in orderto keep current density at a minimum so as to minimize resistivepolarization losses. However, the cell will have low internal impedanceif the ion conduction cross-sectional area of the membrane is too large.

Accordingly, there is a need for a practical way of using available highbarrier, low conductivity membrane materials to provide athermo-electrochemical heat engine that can approximate a Carnotequivalent cycle, that can operate over a wide range of heat sourcetemperatures, and that eliminates the reliability and inefficiencyproblems associated with mechanical engines. The solid state heat engineof the present invention fulfills this need.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention relates to a co-sintered orfused, high density MEA stack or electrochemical cell configured toelectrochemically expand or compress an ionizable working fluid. The MEAstack is a multi-layered structure of alternating thin electrodes andmembranes. The membranes are preferably non-porous and conductive ofions of the working fluid. The membranes are a high diffusion barrier tothe working fluid that has not been ionized. The electrodes arepreferably porous and include additives to promote electronicconductivity and a catalyst to promote the desired electrochemicalreactions.

In one embodiment, the MEA stack is preferably made of ceramic materialsand has a co-sintered structure. In another embodiment, the MEA stack ispreferably made of polymeric materials and has a fused structure.Co-sintering or fusing of the components of the MEA stack allows for apractical construction of a large membrane area within a relativelysmall volume, while avoiding the complications and challenges associatedwith construction of individual cells and then making individualinterconnects including flow manifolds, seals and electricalconnections. The electrochemical cells or MEA stacks of the presentinvention also preferably operate on pressure differentials.

I have also surprisingly discovered that in-plane flow of the workingfluid within thin electrodes enables construction of MEA stacks havinghigh energy conversion density. This is unexpected, considering thatexisting fuel cell art teaches against in-plane flow of reaction fluidswithin thin electrodes. Specifically, existing fuel cell art teachesthat electrodes should be thin, but that flow through the electrodeshould be perpendicular to the ion conductive membrane (i.e., notin-plane), in order to minimize concentration polarization effects.

In operation, working fluid enters an MEA stack through one of theporous electrodes and releases electrons to that electrode as its ionsenter and are conducted through the membrane. The electrons are routedthrough an external circuit to the other electrode on the opposite sideof the membrane. The ions are conducted through the membrane and exitthe electrode on the opposite side. The working fluid is reconstitutedas its ions exit the membrane and recombine with the electrons. The thinelectrodes and membranes are stacked at high density in alternatingsequence with each other, such that adjacent MEA stacks share a commonelectrode. More particularly, the high density MEA stacks of the presentinvention are preferably configured such that each membrane issandwiched by a pair of electrodes, with one of the electrodes of thepair being positioned on the high pressure side of the membrane and theother electrode of the pair being positioned on the low pressure side ofthe membrane.

In another embodiment, the present invention relates to athermo-electrochemical converter, preferably configured as a JTEC,direct heat to electricity engine having a monolithic co-sinteredceramic structure or a monolithic fused polymeric structure. Theco-sintered ceramic structure or fused polymeric structure preferablyincludes a heat exchanger and first and second high density MEA stacksof the structure described above. The heat engine further contains anionizable working fluid that circulates within a continuous flow loopbetween the two high density MEA stacks within a system of high and lowpressure conduits.

The first high density MEA stack is preferably connected to a heatsource and functions to expand the working fluid from a high pressure toa low pressure. The expansion of the working fluid through the first MEAstack generates electricity. The second high density MEA stack ispreferably connected to a heat sink and functions to pump the workingfluid from a low pressure to a high pressure. Electrical power isconsumed by the compression process and the heat of compression isrejected.

The co-sintered or fused heat engine preferably further comprises aconduit system including at least one high pressure flow channel, andmore preferably a plurality of high pressure flow channels, which couplethe flow of the working fluid between high pressure electrodes of thefirst high density MEA stack to high pressure electrodes of the secondhigh density MEA stack, such that the connected high pressure electrodesare essentially at the same pressure. The conduit system preferablyfurther includes at least one low pressure flow channel, and morepreferably a plurality of low pressure flow channels, which couple theflow of the working fluid between the low pressure electrodes of thefirst high density MEA stack to the low pressure electrodes of thesecond high density MEA stack, such that the connected low pressureelectrodes are essentially at the same pressure. The high pressureelectrodes within each high density MEA stack are preferablyelectrically connected to each other. Similarly, the low pressureelectrodes within each high density MEA stack are preferablyelectrically connected to each other. As such, the electricallyconnected high density MEA stacks function as a single membraneelectrode assembly having a large area and a Nernst voltage that is afunction of the stacks' temperature and the pressure differential acrossthe membranes.

In one embodiment, sections of the high pressure channels and sectionsof the low pressure channels are preferably in physical contact witheach other, and thus have a high interface area and thermal conductivityso as to facilitate effective heat transfer between working fluid in ahigh pressure channel and working fluid in a low pressure channel.

The heat exchanger of the co-sintered or fused heat engine preferablyfunctions as a recuperative heat exchanger to recuperate heat fromworking fluid leaving the high temperature MEA stack by coupling it toworking fluid flowing to the high temperature MEA stack. Providing sucha recuperative heat exchanger in combination with a heat source and heatsink coupled to the high and low temperature electrochemical cells(i.e., MEA stacks) enables sufficient heat transfer for near constanttemperature expansion and compression processes, thereby allowing theengine to approximate the thermodynamic Ericsson cycle.

In one embodiment, wherein the MEA stacks operate as part of an engine,the heat source to which the first MEA stack is coupled is preferably atan elevated temperature relative to the temperature of the heat sink towhich the second MEA stack is coupled. As such, the higher temperatureMEA stack (i.e., the first MEA stack) has a higher Nernst voltage thanthe lower temperature stack (i.e., the second MEA stack). The voltagegenerated by the high temperature MEA stack is high enough to overcomethe Nernst voltage of the low temperature MEA stack and have sufficientvoltage left over to power an external load connected in series.

In another embodiment, in which the MEA stacks operate as part of a heatpump application, the first MEA stack is preferably coupled to a heatsource that is at a reduced temperature and the second MEA stack ispreferably coupled to a heat sink that is at an elevated temperaturerelative to the heat source of the first MEA stack. Working fluid isexpanded at a low temperature in the first MEA stack as the heat ofexpansion is extracted from the low temperature heat source. Workingfluid is compressed at a high temperature in the second MEA stack, andthe heat of compression is rejected at the elevated temperature. Becauseof the low temperature of the first MEA stack, the first MEA stackproduces a Nernst voltage that is less than that of the high temperatureMEA stack. An external power source is connected in series with the lowtemperature MEA stack in order to provide a combined voltage that ishigh enough to overcome the Nernst potential of the high temperature MEAstack and thereby drive the compression process therein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of preferred embodiments of thepresent invention will be better understood when read in conjunctionwith the appended drawing. For the purposes of illustrating theinvention, there is shown in the drawing an embodiment which ispresently preferred. It is understood, however, that the invention isnot limited to the precise arrangements and instrumentalities shown. Inthe drawings:

FIG. 1 is a plot of Nernst voltage versus temperature for severalpressure ratios;

FIG. 2 is a diagram of a Johnson Thermo-Electrochemical Converterincluding two membrane electrode assemblies connected back to back by arecuperative heat exchanger;

FIG. 3 is a diagram of the Ericsson thermodynamic cycle;

FIG. 4 is a schematic of a high density co-sintered or fused membraneelectrode assembly stack in accordance with an embodiment of the presentinvention;

FIG. 5 is a schematic of a co-sintered or fused heat engine inaccordance with an embodiment of the present invention;

FIG. 6 is a partial cross-sectional view of the co-sintered or fusedheat engine shown in FIG. 5;

FIG. 7 is a schematic of a heat engine using co-sintered or fusedmembrane electrode assembly stacks in accordance with an embodiment ofthe present invention;

FIG. 8 is a schematic of a heat pump using co-sintered or fused membraneelectrode assembly stacks in accordance with an embodiment of thepresent invention; and

FIG. 9 is a cross-sectional view of a high density co-sintered or fusedmembrane electrode assembly stack in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenienceonly and is not limiting. The words “proximal,” “distal,” “upward,”“downward,” “bottom” and “top” designate directions in the drawings towhich reference is made. The words “inwardly” and “outwardly” refer todirections toward and away from, respectively, a geometric center of thedevice, and designated parts thereof, in accordance with the presentinvention. Unless specifically set forth herein, the terms “a,” “an” and“the” are not limited to one element, but instead should be read asmeaning “at least one.” The terminology includes the words noted above,derivatives thereof and words of similar import.

It will also be understood that terms such as “first,” “second,” and thelike are provided only for purposes of clarity. The elements orcomponents identified by these terms, and the operations thereof, mayeasily be switched.

Referring to the drawings in detail, wherein like numerals indicate likeelements throughout the several views, FIGS. 4-9 show preferredembodiments of a high density co-sintered MEA stack 10. The terms“electrochemical cell,” “membrane electrode assembly stack,” “MEAstack,” and “stack” are used interchangeably herein.

In one embodiment, where components of the high density MEA stack aremade of ceramic materials, the MEA stack 10, and more particularly eachof the flat components of the stack 10 (as described in detailhereinafter), is produced by co-sintering. Co-sintering is a known, lowcost procedure for shaping ceramic materials, and more particularly forthe fabrication of thin (i.e., from 20 μm up to 500 μm) flat components.Co-sintering technology can be used to produce a wide variety ofcontrolled morphologies, from highly porous to fully densemicrostructures. The co-sintering process is well known to those skilledin the art.

Generally, starting powders of different natures, and more particularlystarting ceramic powders, are incorporated and mixed together with anaqueous medium to form a slurry, and the slurry is then cast into greentapes using a tape casting method. Tape casting also allows for stackingthe cast green tapes to obtain a multilayered final product (i.e., theMEA stack 10). More particularly, multiple coating layers of greenceramic material may be cast or screen printed onto each other to form alayered structure which is ultimately sintered to form a MEA stack.

For a given powder, the sintering behavior of the cast green tapes, andhence the final microstructure of the sintered layers, depends on thearrangement and particle sizes, dispersion and homogeneity of thestarting ceramic powder particles in the slurry. Consequently, theslurry formulation is a very important step in the shaping process.

Preferably, the slurry is composed of a mixture of several organic andinorganic compounds. The organic components preferably include a binder,a dispersant, a plasticizer, and, in the case of organic tape casting, asolvent. Other additives, such as wetting agents, defoamers, and poreformers (if porosity is desired in the final microstructure) may also beused to form the slurry. The inorganic compounds include the ceramicpowders to be shaped, sintering additives, and water as themedium/solvent for the aqueous tape casting. An example of a ceramicpowder for formation of a high temperature MEA stack (as discussed inmore detail hereinafter) 10 is yttrium doped barium cerate (Y:BaCeO₃).An example of ceramic powder for formation of a low temperature MEAstack (as discussed in more detail hereinafter) 10 is a composite of 95%LiH₂PO₄ with 5% H₃PO₄.

After casting, the stacked cast tapes are allowed to dry. The tapes maybe allowed to air dry for a predetermined period of time or may bepassed through a drier to accelerate drying. Select organic componentsmay remain in the green tapes after drying. The tapes are then heated toelevated temperatures to effect sintering of the cast green tapes. Theorganic components which remained after drying are sacrificial materialsthat are removed when the tapes are heated for sintering. As such, theremaining organic components give rise to pores and flow passages whichremain during the subsequent sintering treatment. The sintered layers ofthe MEA stack 10 are thus formed.

In another embodiment, where components of the high density MEA stackare made of polymeric materials, the MEA stack 10, and more particularlyeach of the flat components of the stack 10 (as described in detailhereinafter), is produced by a fusing process. The various types offusing processes are well known to those skilled in the art. Forexample, in one type of the fusing process, the polymeric materials maybe softened using a solvent and/or glued together using apolymer/solvent solution. In another type of fusing process, thepolymeric components may be assembled together in a series of hotpressing steps wherein a layer is added and hot pressed in place witheach step.

Referring to FIG. 4, there is shown the internal configuration 6 of ahigh density monolithic MEA stack 10 in accordance with a preferredembodiment of the present invention. The MEA stack 10 comprisesoverlapping layers of alternating electrodes 23 and membranes 22arranged in a high density stacked configuration. That is, each membrane22 is sandwiched between a pair of electrodes 23, such that theelectrodes 23 are stacked in alternating sequence with the membranes 22,thus forming MEA stacks 10 through which a working fluid, preferablyhydrogen, can pass by undergoing an electrochemical oxidation/reductionprocess.

The membranes 22 are preferably ion conductive membranes or protonconductive membranes having a thickness on the order of approximately0.1 μm to 500 μm, and more preferably between approximately 1 μm and 500μm. More particularly, the membranes 22 are preferably made from aproton conductive material, and more preferably a polymer protonconductive material or a ceramic proton conductive material. In oneembodiment, the membranes 22 are preferably formed of a materialcomprising a compound represented by the general formula Na_(x) Al_(y)Ti³⁺ _(x-y) Ti⁴⁺ _(8-x) O₁₆, as disclosed in U.S. Pat. No. 4,927,793 ofHori et al., which is incorporated herein by reference, since thismaterial exhibits high proton conductivity over a broad temperaturerange. However, it will be understood by those skilled in the art thatany material, and preferably any polymer or ceramic material, whichdemonstrates a similar proton conductivity over a broad temperaturerange may be used to form the membranes 22. For example, in an alternateembodiment, the membranes 22 are formed of hydronium beta” alumina. Thepolymer or ceramic membrane material 22 preferably forms a high barrierto molecular working fluid flow and provides for effective containmentof the working fluid.

The electrodes 23 are preferably thin electrodes having a thickness onthe order of approximately 10 μm to 1 cm, and more preferablyapproximately 50 μm to 1,000 μm. The use of different materials for thevarious components (i.e., the electrodes 23 and the membranes 22) couldresult in very high thermal stresses due to differences in the thermalexpansion coefficients between the materials. Accordingly, theelectrodes 23 are preferably comprised or formed of the same material asthe membranes 22. However, the electrodes 23 are preferably porousstructures, while the membranes 22 are preferably non-porous structures.Because the same basic material composition is preferably used for theelectrodes 23 as for the bulk membrane 22 material structure, the highthermal stresses that would otherwise occur under the extremetemperatures encountered during co-sintering or fusing to form the MEAstacks 10 and in many end-use applications during operation of the MEAstacks 10 are eliminated or at least reduced. However, it will beunderstood that the electrodes 23 and the membranes 22 may be formed ofdifferent materials having similar thermal expansion coefficients, suchthat there would be little or no thermal stress generated duringco-sintering/fusing or use of the MEA stack 10.

In one embodiment, the porous electrodes 23 may be doped or infused withadditional material(s) to provide electronic conductivity and catalyticmaterial, in order to promote oxidation and reduction of the workingfluid.

The length 28 of the MEA stack 10 is preferably between approximately0.25 cm and 10 cm. The width (depth into the drawing) of the MEA stack10 is preferably between approximately 1 cm and 100 cm. However, it willbe understood by those skilled in the art that the dimensions of the MEAstack 10 may vary and be selected as appropriate depending on theapplication in which the MEA stack 10 is to be used.

Given the low ion conductivity of known and available ceramic materialswhich may be used to form the membranes 22 and the low Nernst voltagelevels generated at reasonable operating temperatures and pressures bythese ceramic membrane materials, high membrane surface areas aredesirable within the MEA stack 10. Resistive losses associated with highcurrent density, as protons are conducted through the membranes, couldotherwise represent a significant reduction in output voltage andthereby efficiency.

Accordingly, the MEA stack 10 has a high density of overlappingelectrodes 23 and membranes 22, which yields a very high membrane toelectrode interface area within a relatively small stack volume, withthe ion conductive material of the membranes 22 comprising the bulkstructure of the MEA stack 10. More particularly, the bulk area of theMEA stack 10 is occupied by a plurality of the membranes 22. It will beunderstood that the bulk area within a particular stack 10 will dependon the number of membrane 22 and electrode 23 layers, as well as therespective thicknesses of such layers, within a given unit of stackheight. For example, a representative stack 10 having membranes 22 witha thickness of 20 μm sandwiched between 40 μm porous electrodes 23, willhave a total membrane area of 166 cm² per cm³ of stack volume. In oneembodiment, the plurality of membranes 22 are surrounded by an externalhousing 21, which may be made of the same or of a different material asthe membranes 22.

The MEA stack 10 further comprises a conduit system including at leastone low pressure conduit 37 (represented by dashed lines in FIG. 4) andat least one high pressure conduit 38 (represented by solid lines inFIG. 4). Preferably, the conduit system includes a plurality of lowpressure conduits 37 and a plurality of high pressure conduits 38. Asupply of an ionizable gas, preferably hydrogen, is contained within theconduit system as the working fluid.

The low pressure conduits 37 direct the flow of the working fluid (e.g.,hydrogen) in the direction of arrow A, while the high pressure conduits38 direct the flow of the working fluid in the direction of arrow B(i.e., the opposite direction of the low pressure conduits 37 flow). Thelow pressure conduits 37 and high pressure conduits 38 define low andhigh pressure sides of the MEA stack 10. The high pressure side of theMEA stack 10 may be at a pressure of as low as 0.5 psi and as high as3,000 psi. Preferably, the high pressure side of the MEA stack 10 ismaintained at a pressure of approximately 300 psi. The low pressure sideof the MEA stack 10 may be at a pressure of as low as 0.0001 psi and ashigh as 0.3 psi. Preferably, the low pressure side of the MEA stack ismaintained at a pressure of approximately 0.03 psi. A preferred pressureratio of the high pressure side to the low pressure side is 10,000:1.The electrodes 23 in each MEA stack 10 are alternatingly coupled to thehigh pressure and low pressure conduits 38, 37, respectively, such thateach membrane 22 is sandwiched between a first electrode 23 supplied bya high pressure conduit 38 and a second electrode 23 supplied by a lowpressure conduit 37. Accordingly, each membrane 22 is preferablysituated between a high pressure electrode 23 b and a low pressureelectrode 23 a, such that each membrane 22 has a high pressure side anda low pressure side.

First and second terminals 31 and 32 are connected to the electrodes 23of the MEA stack 10. Each terminal 31, 32 is preferably connected to theelectrodes 23 in an alternating sequence, such that the high pressureelectrodes 23 are connected to each other and to one of the terminals(e.g., the first terminal 31) and the low pressure electrodes 23 a areconnected to each other and to the other terminal (e.g., the secondterminal 32).

In one embodiment, the MEA stack 10 may be configured to expand theworking fluid from high pressure to low pressure so as to generateelectricity. Still referring to FIG. 4, power may be extracted from theMEA stack 10 by connecting an electric load to the first and secondterminals 31 and 32. Electric power is produced as the pressuredifferential between the high and low pressure conduits 38, 37 forcesthe working fluid through the MEA stack 10.

Referring to FIG. 1, using a preferred pressure ratio of 10,000:1, wherethe MEA stack 10 is a high temperature stack, operating at a temperatureof 625K, the high temperature MEA stack 10 would have a Nernst voltageof approximately 250 mV. On the other hand, if one maintains operationof the MEA stack 10 at a relatively low temperature of 325K, the lowtemperature MEA stack 10 would have a Nernst voltage of approximately125 mV. In this case, the open circuit voltage of the converter would beapproximately 125 mV.

Referring again to FIG. 4, while under pressure, the working fluid isoxidized at the high pressure electrodes 23 b connected to common secondterminal 32, thereby releasing electrons to the electrodes 23 b andcausing ions of the working fluid to enter the ion/proton conductivemembranes 22 as indicated by arrows 33. With the electrodes 23 bconnected to an external load, electrons flow through the load, throughcommon first terminal 31 and then to the low pressure electrodes 23 a,where ions/protons exiting the membranes 22 are reduced to reconstitutethe working fluid. The converter supplies power to the external load aspressure forces the working fluid to flow through the MEA stack 10. Inone embodiment, a heat source (not shown) may be coupled to the MEAstack 10 to supply heat of expansion to the working fluid so as tomaintain a continuous and nearly isothermal expansion process.

In another embodiment, the MEA stack 10 is configured to operate to pumpthe working fluid from low pressure to high pressure creating acompression process. Electrical power is consumed by the compressionprocess. A power source is applied across the first and second terminals31 and 32. Voltage is applied at a potential that is sufficient to forcecurrent flow by overcoming the Nernst potential generated by the MEAstack 10 at its operating temperature and pressure differential. Theapplied power strips electrons from the working fluid at the interfaceof each low pressure electrode 23 a and membrane 22. The resulting ionsare conducted through the ion conductive membranes 22 in the directionindicated by arrows 39. The power source supplies electrons to the highpressure electrodes 23 b via the first terminal 31, so as toreconstitute the working fluid at the interface of each high pressureelectrode 23 b and membrane 22 as ions exit the membrane 22. Thiscurrent flow under the applied voltage, in effect, provides the pumpingpower needed for pumping the working fluid from low pressure to highpressure. In one embodiment, a heat sink (not shown) may be coupled tothe MEA stack 10 to remove the resulting heat of compression, so as tomaintain a continuous compression process.

Referring to FIG. 5, there is shown a co-sintered/fused high densitydirect heat to electricity converter or heat engine 11, and moreparticularly a monolithic JTEC 11, in accordance with a preferredembodiment of the present invention. The monolithic structure of theJTEC 11 includes a heat exchanger 13, a first high density MEA stack 14,and a second high density MEA stack 16. A first interface 12 is providedfor connection of one of the MEA stacks 14, 16 to a heat sink (therebyforming a low temperature MEA stack) and a second interface 18 isprovided for connection of the other one of the MEA stacks 14, 16 to aheat source (thereby forming a high temperature MEA stack). The firstand second high density MEA stacks 14, 16 generally have the sameconfiguration and structure as described above for the co-sintered MEAstack 10. However, it will be understood that the particular materialemployed as the membranes (i.e., ion conductors) in the high temperaturestack may be different form that employed in the low temperature stack.For example, in one preferred embodiment, the high temperature MEA stack14, 16 is formed of ceramic materials, while the low temperature MEAstack 14, 16 is formed of polymer materials.

Referring to FIGS. 6-8, the first high density MEA stack 14 includes aplurality of porous electrodes 25, 41 and an ion or proton conductivemembrane 24 sandwiched between each pair of adjacent electrodes 25, 41.The second high density MEA stack 16 includes a plurality of porouselectrodes 23, 42 and an ion or proton conductive membrane 22 sandwichedbetween each pair of adjacent electrodes 23, 42. In each stack 14, 16,the porous electrodes 25, 41 and 23, 42 are stacked in alternatingsequence with the membranes 24, 22.

Referring to FIGS. 5-6, the thermo-electrochemical converter 11 alsoincludes a plurality of conduits 37, 38 and an ionizable working fluidcontained within the conduits 37, 38. As discussed above with respect tothe MEA stack 10, conduits 38 are high pressure conduits and conduits 37are low pressure conduits. Preferably, one electrode 41, 42 of anysequential pair of electrodes 25, 41 and 23, 42 is coupled to a highpressure conduit 38 for high pressure flow of the working fluid and theother electrode 23, 25 of the sequential pair of electrodes 25, 41 and23, 42 is coupled to a low pressure conduit 37 for low pressure flow ofthe working fluid. Each of the sandwiched membranes 22, 24 is therebysubjected to the pressure differential between a high pressure porouselectrode 41, 42 and a low pressure porous electrode 23, 25. The highpressure conduits 38 couple high pressure working fluid flow between thehigh pressure electrodes 42 of the second MEA stack 16 and the highpressure electrodes 41 of the first MEA stack 14. Similarly, the lowpressure conduits 37 couple low pressure working fluid flow between thelow pressure electrodes 23 of the second MEA stack 16 and the lowpressure electrodes 25 of the first MEA stack 14.

Referring to FIG. 7, in one embodiment, the thermo-electrochemicalconverter 11 is attached to an external electrical load 56. The firstMEA stack 14 is preferably coupled to an elevated temperature heatsource 58 and the second MEA stack 16 is preferably coupled to a heatsink 60 which operates at a temperature below the elevated temperatureof the first MEA stack 14 and the heat source 58. As such, the first MEAstack 14 is a high temperature stack and the second MEA stack 16 is alow temperature stack. Preferably, the high temperature stack 14 isformed of ceramic materials and the low temperature stack 16 is formedof polymer materials.

The low temperature stack 16 may operate in the range of −50° C. to1,500° C., preferably approximately 55° C. However, the operatingtemperature of the low temperature stack 16 must be sufficiently high soas to have heat efficiently removed from it by ambient temperature air,water or other suitable heat sink in its environment. The hightemperature stack 14 may operate at temperatures between ambient to ashigh as 1,500° C., preferably approximately 550° C. Preferably, the hightemperature stack 14 operates at a higher temperature than the lowtemperature stack 16. It will be understood that, for a heat enginegenerating power, the higher the temperature difference between the twostacks, the greater the engine's theoretical conversion efficiency.Total load 50, which consists of the external load 56 and the second MEAstack 16 connected in series, is coupled to the first MEA stack 14 byfirst and second terminals 52 and 54.

Still referring to FIG. 7, the first MEA stack 14 supplies power to thetotal load 50 as pressure forces flow of the working fluid from thefirst set of high pressure electrodes 41, which are connected to aterminal 35, to the first set of low pressure electrodes 25, which areconnected to a terminal 34. The power is supplied at the Nernst voltageof the first MEA stack 14 based on the applied pressure differential andits temperature less the voltage loss due to the internal impedance ofthe stack 14. Under the force of pressure, electrons are conductedthrough total load 50 and ions 36 are conducted through the ionconductive membranes 24 of the first MEA stack 14.

The voltage produced by the first MEA stack 14 is divided between thesecond MEA stack 16 and the external load 56 of total load 50. Asconfigured, a portion of the power produced by the first MEA stack 14 issupplied to the second MEA stack 16 by connection to a second set ofhigh pressure electrodes 42 (i.e., the electrodes connected to theterminal 31) of the second MEA stack 16 and a second set of low pressureelectrodes 23 (i.e., the electrodes connected to the terminal 32).Working fluid flow is forced from low pressure to high pressure as theelectron flow forced under the applied power induces ion conductivitythrough the ion conductive membranes 22 of the second MEA stack 16. Theremaining power produced by the first MEA stack 14 is supplied toexternal load 56.

The thermo-electrochemical converter 11′ shown in FIG. 8 is alsoconfigured as a JTEC and operates as a heat pump. Essentially, theoperation of the thermo-electrochemical converter 11′ is in reverse tothat of the engine 11 of FIG. 7. Referring to FIG. 8, the heat pump 11′is attached to an external power source 58. The first MEA stack 14 iscoupled to a low temperature heat source 66 and operates to remove heatfrom the low temperature heat source 66, thereby creating arefrigeration effect. The low temperature heat source 66 may operate ata temperature of −50° C. to 100° C., and preferably operates at atemperature of approximately 15° C. More preferably, the low temperatureheat source 66 operates at a temperature which is sufficiently low tohave heat of working fluid expansion effectively transferred to the lowtemperature heat source 66 from ambient temperature air or water orother suitable heat source in its environment where cooling is desired.The first MEA stack 14 generates power as pressure forces working fluidflow from the first set of high pressure electrodes 41, which areconnected to the terminal 35, to the first set of low pressureelectrodes 25, which are connected to the terminal 34. Pressure forcesworking fluid flow through the first MEA stack 14 by forcing ionconduction through ion conductive membranes 22 of the first MEA stack14, as electrons are conducted through the external power source 58 andthe second MEA stack 16 in series.

Still referring to FIG. 8, the external power source 58 and the firstMEA stack 14 are connected in series and comprise a total power source61. The total power source 61 supplies power to the second MEA stack 16and is connected to the second set of high pressure electrodes 41 (i.e.,the electrodes connected to terminal 31) and the second set of lowpressure electrodes 23 (i.e., the electrodes connected to terminal 32)within the second MEA stack 16. The working fluid is transferred fromthe low pressure electrodes 23 to the high pressure electrodes 41 aselectron flow forced by the power source 61 induces working fluid ionconduction through the ion conductive membranes 22 of the second MEAstack 16. The first MEA stack 14 is coupled to first and secondterminals 62 and 64.

Still referring to FIG. 8, the second MEA stack 16 is coupled to anelevated temperature heat sink 68 and thus operates at highertemperature than the first MEA stack 14. The second MEA stack 16 rejectsthe heat of compression of the working fluid to the high temperatureheat sink 68 for effective operation of the engine as a heat pump 11′.The Nernst voltage of the high temperature second MEA stack 16 is higherthan the Nernst voltage of the low temperature first MEA stack 14. Theadditional voltage needed to overcome the higher Nernst voltage of thehigh temperature second MEA stack 16 is provided by the external powersource 58.

The monolithic structure of the high density direct heat to electricityconverter or heat engine 11, and more particularly the co-sintered orfused monolithic structure of the MEA stacks 14, 16, results in a moreefficient engine construction process, as compared with conventionalconverters. This is because the need to make many tedious interconnectsand, more importantly, the need to construct thicker and more bulkystandalone electrode and membrane layers are unnecessary. For example, amembrane of a conventional converter typically has a thickness on theorder of 100 μm in order to have sufficient integrity to survive theconstruction process. In the converter 11 of the invention, themembranes 22 may be thin coatings on the order of 10 μm or less. Thesequential coating of multiple thin layers onto each other or thesequential lamination of multiple thin layer to each other results in amonolithic multilayered structure, wherein the layers mechanicallyreinforce each other to provide structural integrity while at the sametime providing high MEA surface area within a relative small MEA stackvolume. The high surface area enables relatively low current density andthereby low resistive losses.

The invention is equivalent to taking fixed source current and dividingit among many high impedance resistors connected in parallel, so thatthe net result is equivalent to a single low impedance resistor.However, the necessary structure and connections are constructed in avery efficient manner, using screen printing or other suitabletechniques to apply multiple coating layers of green ceramic materialonto each other and then sintering them into a solid state heat enginehaving single monolithic structure or using suitable techniques to fusetogether multiple polymer foils, films or layers into a solid state heatengine having single monolithic structure.

Referring to FIG. 9, there is shown a cross-sectional view of amonolithic MEA stack 80 including an outer casing 90, proton conductivemembrane material 93, high pressure porous electrodes 94, low pressureporous electrodes 95, high pressure flow conduits 97 and low pressureflow conduits 96. The MEA stack 80 is configured such that high pressureworking fluid flowing within the high pressure conduits 97 can freelyflow into or out of the high pressure porous electrodes 94. The highpressure porous electrodes 94 are electrically connected to each otherby an interconnect 98. Similarly, working fluid flowing within the lowpressure conduits 96 can easily flow can into or out of the low pressureporous electrodes 95. The low pressure porous electrodes 96 areconnected to each other by an interconnect 99. The interconnects 98 and99 may or may not be porous. Non-porous electrical terminals 91 (seealso FIG. 5) and 92 provide electrical contact points for externalelectrical connections to the MEA stack 80. Terminal 91 is connected tothe low pressure porous electrodes 95 and terminal 92 is connected tothe high pressure porous electrodes 94. As FIG. 9 illustrates, withconstituent MEAs electrically connected in parallel, connections maysimilarly be made to connect individual MEA stacks in series forincreased net output voltage.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

I claim:
 1. A heat to electricity converter comprising: at least threeporous electrodes; a working fluid; at least two ion or protonconductive membranes, the at least three porous electrodes being stackedwith the at least two ion or proton conductive membranes in anoverlapping configuration whereby each ion or proton conductivemembranes membrane is sandwiched between a pair of porous electrodes,the at least three porous electrodes and the at least two ion or protonconductive membranes comprising a co-sintered or fused monolithicstructure; and a first conduit being a high pressure conduit containingthe working fluid at a first pressure and a second conduit being a lowpressure conduit containing the working fluid at a second pressure whichis lower than the first pressure, wherein a first porous electrode ofeach sequential pair of the porous electrodes is coupled to the highpressure conduit for high pressure working fluid flow therethrough and asecond porous electrode of each sequential pair of the porous electrodesis coupled to the low pressure conduit for low pressure working fluidflow therethrough, the first porous electrode being a high pressureelectrode and the second porous electrode being a low pressureelectrode, and wherein the converter generates electrical power when anelectrical load is connected between the high and low pressureelectrodes with the converter being coupled to a heat source, heat fromthe heat source being converted into electrical power as the workingfluid expands from high pressure to low pressure with the heat ofexpansion being supplied by the heat source, or wherein the converterfunctions as a heat pump when electrical power is connected between thehigh and low pressure electrodes with the converter being coupled to aheat sink, a heat of compression being rejected to the heat sink as theelectrical power drives compression of the working fluid from lowpressure to high pressure.
 2. The heat to electricity converteraccording to claim 1, further comprising an external power source, thepower source being coupled between each sequential pair of porouselectrodes whereby power applied to the at least three porous electrodesforces a flow of the working fluid flow from the low pressure electrodeto the high pressure electrode as electron flow is forced by theexternal power source and induces ion or proton conductivity through themembrane sandwiched between the sequential pair of porous electrodes. 3.The heat to electricity converter according to claim 1, furthercomprising an electrical load, the electrical load being coupled betweeneach sequential pair of porous electrodes whereby pressure forces flowof the working fluid from the high pressure electrode to the lowpressure electrode by means of electrons being conducted through theexternal load and ions or protons being conducted through the membranesandwiched between the sequential pair of porous electrodes.
 4. The heatto electricity converter according to claim 1, wherein each of the atleast two ion or proton conductive membranes is formed of a ceramicconductive material or a polymer conductive material.
 5. The heat toelectricity converter according to claim 1, wherein the at least threeporous electrodes are electrically connected in series with each other.6. The heat to electricity converter according to claim 1, furthercomprising a recuperative heat exchanger, the heat exchanger includingthe high and low pressure conduits configured to enable transfer of heatbetween the high pressure fluid and the low pressure fluid.